Here is a link to the largest private home ever built in the USA. It was built over 100 years ago by the grandson of Cornelius Vanderbilt, George Washington Vanderbilt. George kept the true cost of construction and decorating a secret. Estimates for building the four acre home in today's dollars start around one billion dollars. The home was built as a summer retreat as the Vanderbilt's family home was in New York. The two main floors can be accessed by an elevator, but the other floors have many steep stairs and tight openings.
Biltmore Estate Ashville North Carolina